The present invention relates to a semiconductor device including a layer of a gallium nitride (GaN) compound semiconductor generally represented by InxAlyGa1−x−yN (0≦X<1, 0≦Y<1, 0≦X+Y<1) (hereinafter, compound semiconductors of this group are collectively called as “GaN compound semiconductors”) and a method for fabricating such a semiconductor device. More particularly, the present invention relates to a Schottky electrode formed in contact with a GaN compound semiconductor layer and a method for forming such a Schottky electrode.
GaN compound semiconductors such as GaN, AlGaN, InGaN, and InAlGaN are direct transition semiconductors having a band gap varying in the range from 1.95 eV to 6 eV. These semiconductors are therefore expected as promising materials for light emitting devices such as laser diodes. GaN is also expected as a promising material for a high-frequency power device since it possesses high dielectric breakdown electric field intensity, high thermal conductivity, and a high electron saturation rate. In particular, an AlGaN/GaN hetero-junction structure has an electric field intensity as high as 1×105 V/cm and an electron velocity twice or more as high as that of GaAs. This structure is therefore expected to contribute to realization of high-frequency operation in combination with miniaturization of the device.
GaN compound semiconductors exhibit n-type characteristics when they are doped with an n-type dopant such as Si and Ge. Therefore, it has been attempted to apply the GaN compound semiconductors to field effect transistors (FETs). In general, a metal semiconductor field effect transistor (MESFET) using a Schottky metal as a Schottky electrode has been studied. Schottky characteristics greatly influence the drain breakdown voltage and the current characteristics of a FET obtained when the gate voltage applied is positive. Conventionally, therefore, in consideration of the Schottky characteristics, metal such as palladium and platinum is generally used as the Schottky electrode for a GaN compound semiconductor.
However, although the metal such as palladium and platinum is good in barrier height and ideal factor n value as indicators of the Schottky characteristics, it is poor in adhesion to a GaN compound semiconductor that is to form a Schottky junction together with the metal. As a result, the electrode is disadvantageously peeled off or lifts during fabrication process. Therefore, for a high-frequency device where a fine gate having a gate length of a sub-half micron is indispensable, in particular, processing of such a Schottky metal will be further difficult.